Bleaching



as i 4 United States Patent BLEACHING Charles J. Rogers, Wilmington, Del., assignor to E. I. du Pont de Nemonrs and Company, Wilmington, Del., a corporation of Delaware No Drawing. Application June 1, 1955 Serial No. 512,587

1 Claim. (Cl. 8-108) This invention relates to fluid treatment of textiles, particularly bleaching of nylon.

Aqueous acid solutions containing chlorite are known to be elfective bleaches for textiles generally, but such solutions customarily evolve considerable chlorine dioxide and seriously corrode most metals. Textiles comprised of linear condensation polymers promise increasing application of this type of bleach except for these disadvantages encountered in its use.

A primary object of the present invention is provision of an effective bleaching composition that is safe to use and non-corrosive to metals. A particular object is improved bleaching of nylon. Other objects, together with means and methods for attaining the various objects will be apparent from the following description of the invention.

In general, the objects of the present invention are accomplished by means of a bleaching composition containing citric acid and both alkali-metal chlorite and nitrate; the composition conveniently includes also a buffer for maintaining an aqueous solution of the composition mildly acid. In aqueous bleaches utilizing this composition, the concentration of alkali-metal chlorite most suitably ranges from several parts per thousand to several parts per ten thousand based upon the weight of water present; the composition itself contains a slight molar excess of citric acid and a somewhat greater (e.g., several mols) excess of alkali-metal nitrate per mol of the chlorite. The material to be bleached is treated conveniently with the solution at a temperature above ordinary room temperature but below the normal boiling temperature of water, preferably in the presence of a suitable detergent. The invention is exemplified below in illustrative detail, parts being by weight unless otherwise indicated.

Example I A composition is made up of 0.1 part sodium chlorite, 0.3 part citric acid, 0.4 part sodium nitrate, 0.2 part disodium hydrogen phosphate (anhydrous basis), and 0.04 part sodium Lorol sulfate. The composition is dissolved in 100 parts water, giving a pH of 4.5. In 400 grams of this solution in a glass container is placed an undyed 10-gram swatch of 96 x 72 fabric woven from 70-denier, 34-filament nylon (polyhexamethylene adiparnide) yarn. Before treatment, this fabric is visibly discolored, giving a differential color value of 8.6 determined according to the instrumental method described by L. G. Glasser and D. J. Troy in J. Optical Soc. Am. 42, 652 (1952). Also placed in the container are small metal tags, one each of aluminum, stainless steel, and Monel metal. After immersion in the solution at 70 C. for two hours with occasional agitation, during which time a trace of chlorine dioxide is detectable in the atmosphere above the surface of the solution, the fabric becomes considerably whiter in appearance, giving when rinsed and dried a color value of 4.5 upon the same basis of measurement as above. Tensile strength of the fabric is decreased slightly (several percent) by the treat "ice ment, but no other change is noticed. Only the Monel metal shows any significant corrosion, evident as a slight pitting of the surface. In three hours at 70 C. the solution evolves a volume of 5.6 milliliters of gaseous chlorine dioxide per liter of solution.

Repetition of the procedure of the above example with the citric acid replaced by a like amount of adipic acid gave equivalent bleaching but produced slight corrosion of the aluminum and moderate corrosion of the Monel metal. Repetition with the citric acid of the example replaced by half as much benzoic more than tripled the evolution of chlorine dioxide, compared with that of the example, and corroded the Monel metal moderately; this amount of chlorine dioxide is within 6 percent of the amount evolving in the same time from a like amount of solution at the same temperature (70 C.) consisting of 0.1 part sodium chlorite in parts water acidified to a pH of 4.5 by acetic acid.

A solution like that of Example I but lacking sodium nitrate was as effective a bleach, but it produced marked corrosion on all three of the test metals, ranging from slight on the stainless steel to moderate on the aluminum and severe on the Monel metal, resulting in easily detectable loss in weight (about one-half percent). When only half the amount of sodium nitrate of the example was present, the corrosion produced was intermediate between that of the example and that of the control containing no sodium nitrate, being noticeable on the stainless steel and moderate on both the aluminum and the Monel metal. Excess of sodium nitrate beyond the amount of the example was found to give results like those of the example in a similar run. Repetition of the same procedure without sodium nitrate and with the additional change of substituting 0.4 part adipic acid for the 0.3 part citric acid produced general corrosion, with weight loss, of all three test metals; addition of 0.1 part sodium nitrate to this bleaching composition markedly reduced the Weight loss, but pitting remained moderate upon both the aluminum and the Monel metal.

Example II A bleaching solution made up of 0.2 part sodium chlorite, 1.3 parts citric acid, 1.6 parts sodium nitrate, and 0.75 part disodium hydrogen phosphate in 100 parts water is employed like the solution of Example I. This solution provides the same results of satisfactory bleaching of nylon fabric and substantial lack of corrosion of the test metals.

The above examples show the eflicacy of the bleaching composition of this invention upon polyhexamethylene adipamide, the nylon polymer most prevalent in commerce; however, it is similarly effective upon polycaproamide and polyhexamethylene sebacamide, for example, and upon linear condensation polymers generally, includin polyethylene terephthalate. This invention is applicable also to bleaching of acrylonitrile polymers, including the homopolymer polyacrylonitrile.

The examples indicate the effectiveness of citric acid in amounts from about 3 to 6 times the weight of sodium chlorite and of sodium nitrate in amounts from about 4 to 8 times the weight of the sodium chlorite. Other alkali metals may be used as a basis for either or both the chlorite and the nitrate, although they (e.g., lithium, potassium) usually will be more costly. Chlorites of the alkaline-earth metals may be suitable as the active bleaching ingredient also. When sodium chlorite is the sole bleaching ingredient of the composition, a solution of the composition for use according to this invention normally will contain from about 0.03% to about 0.2% of sodium chlorite by weight; other of the mentioned inorganic chlorites may be substituted proportionately, and

additional chlorite may be added to the solution during use, if desired.

The desired mildly acid condition (i.e., pH of from about 4.5 to about 5.5) of the solution may be provided by buffers other than disodium hydrogen phosphate, of: course; some examples of suitable bufiering compounds are diammonium hydrogen phosphate, potassium acid phthalate, and mucic acid. Operation at stronger acidity is undesirable because it it conducive to increased evolution of chlorine dioxide; the examples are drawn particularly to the low end of the specified pH range where more serious corrosion would be expected. In addition to a suitable buffer, polyhydroxylated compounds, e.g., dextrose, sucrose, and sorbitol, may be added to assist in repressing release of chlorine dioxide.

With most textiles, addition of a neutral anionic or non-ionic detergent is desirable in order to facilitate access of the bleaching agent to the surface of the textile. For most purposes, an amount of solution from about 20 to about 50 times the weight of the textile will be provided, and the concentration of the detergent will be on the order of only a few parts per ten thousand of the solution. Among suitable detergents may be mentioned sodium 2-methylpentane-4-sulfate, sodium sulfoethylmethyllauramide, sodium sulfoethyl myristate, sodium diamyl sulfosuccinate, sodium dodecylbenzene sulfonate, and sodium hexadecyl sulfonate.

The advantages of the effective bleaching and the reduced corrosion provided by the present invention have been mentioned above. Of course, the bleaching composition itself may be handled and stored easily in the solid state and only need be mixed with water to prepare the bleaching solution; the bufier may be added conveniently to either the solid ingredients or the solution. Substantial reduction in evolution of gaseous chlorine dioxide from the bleaching solution renders the working area safer for operating personnel. Other benefits will be ap parent to persons undertaking to practice this invention.

The claimed invention:

In the process of bleachin textile material comprising nylon in a container composed substantially of a metal from the group consisting of stainless steel, aluminum, and Monel metal, in an acidic aqueous solution containing from about 0.03% to about 0.2% by weight of an alkali-metal chlorite, said process being characterized by the evolution of chlorine dioxide, the improvement comprising suppressing the evolution of the chlorine dioxide by dissolving in said alkali-metal chlorite solution from about 3 to about 6 parts of citric acid and from about 4 to about 8 parts of alkali-metal nitrate for every one part of alkali-metal chlorite, and bufiering the solution at a pH in the range of from about 4.5 to about 5.5 with disodium hydrogen phosphate while maintaining said solution at a temperature above ordinary room temperature and below the normal boiling point of water.

References Cited in the file of this patent UNITED STATES PATENTS 2,071,091 Taylor Feb. 16, 1937 2,121,501 Hershman June 21, 1938 2,236,970 Goldfarb Apr. 1, 1941 2,353,210 Williams July 11, 1944 2,526,839 Aston Oct. 24, 1950 2,691,637 Waibel Oct. 12, 1954 2,710,792 McDonald et al. June 14, 1955 2,739,032 Wilson Mar. 20, 1956 

